The present invention relates to the art of diagnostic nuclear imaging. It finds particular application in conjunction with gamma cameras and single photon emission computed tomography (SPECT), and will be described with particular reference thereto. However, it is to be appreciated that the present invention is also amenable to other like applications.
Diagnostic nuclear imaging, is used to study a radionuclide distribution in a subject. Typically, in SPECT, one or more radiopharmaceuticals or radioisotopes are injected into a subject. The radiopharmaceuticals are commonly injected into the subject""s blood stream for imaging the circulatory system or for imaging specific organs which absorb the injected radiopharmaceuticals. One or more gamma or scintillation camera detector heads, typically including a collimator, are placed adjacent to a surface of the subject to monitor and record emitted radiation. The camera heads typically include a scintillation crystal which produces a flash or scintillation of light each time it is struck by radiation emanating from the radioactive dye in the subject. An array of photomultiplier tubes and associated circuitry produce an output signal which is indicative of the (x, y) position of each scintillation on the crystal. Often, the heads are rotated or indexed around the subject to monitor the emitted radiation from a plurality of directions to obtain a plurality of different views. The monitored radiation data from the plurality of views is reconstructed into a three dimensional (3D) image representation of the radiopharmaceutical distribution within the subject.
One of the problems with this imaging technique is that photon absorption and scatter by portions of the subject between the emitting radionuclide and the camera head distort the resultant image. One solution for compensating for photon attenuation is to assume uniform photon attenuation throughout the subject. That is, the subject is assumed to be completely homogenous in terms of radiation attenuation with no distinction made for bone, soft tissue lung, etc. This enables attenuation estimates to be made based on the surface contour of the subject. Of course, human subjects do not cause uniform radiation attenuation, especially in the chest.
In order to obtain more accurate radiation attenuation measurements, a direct measurement is made using transmission computed tomography techniques. In this technique, radiation is projected from a radiation source through the subject. The transmission radiation is received by detectors at the opposite side. The source and detectors are rotated to collect transmission data concurrently with the emission data through a multiplicity of angles. This transmission data is reconstructed into an image representation or non-uniform attenuation map using conventional tomography algorithms. The radiation attenuation properties of the subject from the transmission computed tomography image are used to correct for radiation attenuation in the emission data. See, for example, U.S. Pat. Nos. 5,210,421 and 5,559,335, commonly assigned and incorporated herein by reference.
However, transmission computed tomography techniques suffer from their own drawbacks. One such drawback is an undesirable increase in the patient""s exposure to radiation due to the transmission scan. Moreover, the transmission scan increase the costs associated with producing clinical SPECT images.
Additionally, the truncation of transmission data or transmission projections due to a relatively small detector size is a well known problem in SPECT. This problem is further exacerbated during transmission imaging of the chest by a three-detector SPECT system with fan-beam collimators. See, for example, G. T. Gullberg, et al., xe2x80x9cReview of Convergent Beam Tomography in Single Photon Emission Computed Tomography,xe2x80x9d Phys. Med. Biol., Vol. 37, No. 3, pp. 507-534, 1992. The truncation problem results in solving a rank deficient system of linear equations, which leads to reconstruction artifacts when common reconstruction algorithms are applied. See, for example: J. C. Gore, et al., xe2x80x9cThe Reconstruction of Objects from Incomplete Projections,xe2x80x9d Med. Phys., Vol. 25, No. 1, pp. 129-136, 1980; G. L. Zeng, et al., xe2x80x9cA Study of Reconstruction Artifacts in Cone Beam Tomography Using Filtered Backprojection and Iterative EM Algorithms,xe2x80x9d IEEE Trans. Nucl. Sci., Vol. 37, No. 2, pp. 759-767, 1990; S. H. Manglos, xe2x80x9cTruncation Artifact Suppression in Cone-Beam Radionuclide Transmission CT Using Maximum Likelihood Techniques: Evaluation with Human Subjects,xe2x80x9d Phys. Med. Biol., Vol. 37, No. 3, pp. 549-5562, 1992; G. L. Zeng, et al., xe2x80x9cNew Approaches to Reconstructing Truncated Projections in Cardiac Fan Beam and Cone Beam Tomography,xe2x80x9d J. Nucl. Med., Vol. 31, No. 5, p. 867, 1990 (abstract); and, B. M. W. Tsui, et al., xe2x80x9cCardiac SPECT Reconstructions with Truncated Projections in Different SPECT System Designs,xe2x80x9d J. Nucl. Med., Vol. 33, No. 5, p. 831, 1992 (abstract).
The present invention contemplates a new and improved technique for SPECT imaging which overcomes the above-referenced problems and others.
In accordance with one aspect of the present invention, a method of constructing a non-uniform attenuation map of a subject for use in image reconstruction of SPECT data is provided. It includes collecting a population of a priori transmission images. The transmission images are not of the subject. A cross-correlation matrix is generated from the population of transmission images. Next, the eigenvectors of the cross-correlation matrix are calculated. A set of orthonormal basis vectors is generated from the eigenvectors. Finally, a linear combination of the basis vectors is constructed, and coefficients for the basis vectors are determined such that the linear combination thereof defines the non-uniform attenuation map.
In accordance with a more limited aspect of the present invention, a Karhunen-Loxc3xa8ve transform is employed to calculate the eigenvectors of the cross-correlation matrix.
In accordance with a more limited aspect of the present invention, the set of orthonormal basis vectors is constructed from a predetermined number of selected eigenvectors chosen from the eigenvectors of the cross-correlation matrix. The selected eigenvectors have corresponding eigenvalues larger than eigenvalues of non-selected eigenvectors.
In accordance with a more limited aspect of the present invention, the step of determining coefficients for the basis vectors includes iteratively comparing projections of the set of orthonormal basis vectors having estimated coefficients with truncated transmission projections from the subject. Using a least-squares fit, coefficients are selected which best match the projections of the set of orthonormal basis vectors to the truncated transmission projections from the subject.
In accordance with a more limited aspect of the present invention, the step of determining coefficients for the basis vectors includes iteratively employing Natterer""s data consistency conditions to relate emission data from the subject to transmission projections. The transmission projections are generated from projections of the set of orthonormal basis vectors having estimated coefficients. Using a least-squares fit, coefficients are selected which generate the transmission projections that best fulfill Natterer""s data consistency conditions.
In accordance with a more limited aspect of the present invention, no transmission scan of the subject is performed.
In accordance with a more limited aspect of the present invention, the predetermined number of selected eigenvectors is less than or equal to approximately 15% of all the eigenvectors.
In accordance with another aspect of the present invention, an image processor for reconstructing images of a distribution of radioactive material in a patient being examined with a gamma camera is provided. It includes an emission memory which stores emission data collected by the gamma camera. An attenuation factor memory stores attenuation factors calculated from a non-uniform attenuation map. A data processor takes the emission data and corrects it for attenuation in accordance with the attenuation factors stored in the attenuation factor memory. A reconstruction processor takes corrected emission data from the data processor and therefrom reconstructs an image representation of the distribution of radioactive material in the patient. An a priori image memory stores a priori transmission data from a plurality of a priori transmission scans of a region of interest that is the same as that being reconstructed. The transmission scans originate from subjects other than the patient. A cross-correlation data processor constructs a cross-correlation matrix from the a priori transmission scans, and an eigenvector data processor calculates eigenvectors of the cross-correlation matrix. A basis data processor constructs a set of orthonormal basis vectors from the eigenvectors of the cross-correlation matrix, and an iterative data processor computes coefficients for the basis vectors such that a linear combination thereof defines the non-uniform attenuation map.
One advantage of the present invention is that accurate non-uniform attenuation maps are achieved.
Another advantage of the present invention is the patients"" radiation dosage is lessened by the elimination of transmission scans.
Yet another advantage of the present invention is that accurate non-uniform attenuation maps are achieved from truncated transmission scans.
Still further advantages and benefits of the present invention will become apparent to those of ordinary skill in the art upon reading and understanding the following detailed description of the preferred embodiments.